A great deal of interest has surrounded the use of monoclonal antibodies (mAbs) for the selective delivery of cytotoxic agents to tumor cells. While a number of different drug classes have been tried for delivery via antibodies, only a few drug classes have proved efficacious as antibody drug conjugates, while having a suitable toxicity profile. One such class is the auristatins, derivatives of the natural product dolastatin 10. Representative auristatins include MMAE (N-methylvaline-valine-dolaisoleuine-dolaproine-norephedrine) and MMAF (N-methylvaline-valine-dolaisoleuine-dolaproine-phenylalanine).
Conjugation of drugs to antibodies, either directly or via linkers, involves a consideration of a variety of factors, including the identity and location of the chemical group for conjugation of the drug, the mechanism of drug release, the structural elements providing drug release, and the structural modification to the released free drug. In addition, if the drug is to be released after antibody internalization, the mechanism of drug release must be consonant with the intracellular trafficking of the conjugate.
MMAF is relatively non-toxic as a free drug, yet is highly potent in activity when conjugated to a mAb and internalized. MMAF has been successfully conjugated to a mAb at the N-terminal amino acid of MMAF via a cathepsin B cleavable peptide linker maleimidocaproyl-valine-citrulline (mc-vc-) and a self-immolative group p-aminobenzyl-carbamoyl (PABC) to produce antibody-linker-drug conjugates of the following structure rnAb-mc-vc-PABC-MMAF. Upon cleavage of the peptide linker, the self-immolative PABC group releases itself from MMAF, liberating free drug.
MMAF was also found to be active as non-cleavable drug linker conjugate, maleimidocaproyl MMAF (mcMMAF). For mcMMAF the maleimidocaproyl and a cysteine from the antibody remain attached to the N-terminus of MMAF.
There remains a need, however, for drug delivery vehicles for the selective release of drug to cells.